The invention concerns a method for the determination of the absolute number of electron spins in a sample of extended size with the assistance of an apparatus for the measurement of magnetic resonance, wherein the extended sample is disposed within a measuring volume of a radio frequency (RF) resonator in an electron spin resonance (ESR) apparatus.
A method of this kind is known e.g. from J. A. Weil et. al., Electron Paramagnetic Resonance, John Wiley and Sons, New York, 1994.
The present invention concerns the measurement of samples using electron spin resonance (ESR). ESR spectroscopy is a method of analytic instrumentation for the investigation of a sample. The sample must thereby have unpaired electrons. Microwave (generally with constant frequency), are irradiated into the sample which is located in a strong magnetic field B0 (the magnetic field B0 is generally modulated).
The chemical composition of the sample can be determined by the absorption properties thereof. The position of the absorption lines can reliably indicate the type of chemical bindings or the associated substances. It is however, often useful to determine the fraction of a particular substance within the sample. In order to do so using ESR spectroscopy, it is necessary to measure the number of electron spins in the sample.
In accordance with J. Weil loc. cit., an absolute quantification can be determined as follows. In addition to the (unknown) sample being measured, a reference sample is prepared which has a known absolute number of electron spins (for example a certain weight amount of a paramagnetic salt) which is otherwise equivalent to the unknown sample, in particular with respect to the type and the sample container as well as with respect to the filling level within the sample container. The required equivalence of the samples with respect to ESR specific properties, include e.g. a so-called saturation behavior and the spectral region in which the ESR lines occur. The unknown sample and the reference sample are thereby measured under the same conditions using ESR. In order to establish equivalent ESR measuring conditions, a so-called double resonator is utilized which has two separate measuring chambers in which the two samples can be inserted. The measurement of the two samples thereby occurs without direct sequential exchange of the sample. This ensures that both samples are measured with the same Q-factor. A double resonator of this kind is e.g. known from Bruker BioSpin GmbH, Rheinstetten, DE, and entitled “ER4105DR EPR resonator”. The absolute number of spins in the unknown sample can be derived from the ratio of the respectively obtained ESR amplitudes.
This method has the disadvantage that it is necessary to prepare and measure a similar type of reference sample for each unknown sample. Moreover, the double resonator is difficult and expensive from a constructional point of view. In addition, the same kind of reference samples can only be prepared for liquid samples. However, ESR samples occur in all kinds of states: in powder form as well as in single crystal form.
J. A. Weil also discloses a procedure for relative quantification of electron spins with which ratios between the number of electron spins of a series of similar samples can be determined. Towards this end, a marker probe sample is measured simultaneously with each sample of the series (e.g. the sample of the series and the marker sample are disposed and measured simultaneously in the same resonator. The marker sample produces its own signal (marker) in the absorption spectrum, which must be easily distinguished from the actual signal of the sample of the series (sample signal). The underlying problem with this procedure is finding a suitable marker. Through determination of the integral intensity of the markers, it is possible to normalize the absorption spectrum of the samples in the series as a result of which the different sample signals are quantitatively comparable. This procedure does not lead to determination of the absolute number of electron spins of a sample in the series.
It is the object of the present invention to present a simplified method for the absolute determination of the number of electron spins in a sample, in particular, wherein it is not necessary to simultaneously measure a similar reference sample for each sample being measured.